Restoring a 1968 Morris Cooper S and wondering what piston compression ratio is best these days.
Let me say first this project started in 1994.
I'm almost complete and now having some second thoughts on the pistons I have used.
I decided some 15 years ago when rebuilding the motor to go with Hepolite 8.8:1cr pistons.
At top dead center they are a good 0.2" below the top of the block deck.
The block was bored to + 60thou (had a seized piston), the head was faced and the block deck faced.
My reasoning at the time was with the increase in swept volume with +60thou pistons, head faced and block faced the compression ratio would be near 9.1:1CR and given fuel octain was a max of 95 at that time.

Interested to hear from others if this was a good idea or; should I replace the pistons with 9.75:1cr?
How much will the reduced compression ratio affect the performance?
The motor is pretty standard otherwise, 1.25" twin SU, Maniflow Cooper Freeflow manifold and RC40, 510 Cam, Aldon distributor.


Ian

Are you sure the block has a deck height of .200" ?

Standard 1275 engines had about .065".

Hepolite standardized the dish in their 21251 (8.8:1) and 21253 (9.75:1) metric pistons.

To reduce the comp ration on the 21251 pistons they increased the deck height of the pistons but I would have thought it would have been only about .040".

You need to measure what deck height you have now to be able to work out what compression ratio you have.

Hope this helps.